This invention relates to coatings for the reduction of reflective cross sections of metallic surfaces, and in particular, to a compact, wide band radar absorptive coating preferably having tuning and control capabilities.
Various absorptive materials have been used in the past to reduce radar cross section of selective objects. However, the materials used in the past, and their application, have been dependent upon the wave length of the incident wave. That is to say, the absorptive qualities of previously known materials have required a considerable thickness of the material in order to perform its function. In general, known prior art material provides a maximum reduction of radar cross section of about 20 db per wave length thickness of the material utilized for absorptive purposes. Known materials also are functional only in a very narrow band of radar frequencies. Consequently, the usefulness of the absorptive materials is exceedingly limited. For example, they are not suitable for use on high performance aircraft because the thickness of material required to make the aircraft non-reflective is too great. That is to say, the thickness required to offer a significant reduction in the reflective cross section of the aircraft is so great as to render its application to the aircraft impractical, particularly in view of the operation speeds of such aircraft.
The invention disclosed hereinafter overcomes these prior deficiencies by providing a material structure which is capable both of passive and active application as a absorptive coating. The coating of this invention exhibits wide band performance and is theoretically capable of reducing radar cross section by approximately 40 db. At high frequencies, the material may be used in its passive mode, a relatively thin thickness of material alone being sufficent to provide the absorptive qualities. At low frequencies, means are provided for injecting a control voltage within the coating, thereby enabling the material to provide absorptive properties, even where the thickness of the applied coating is less than the wave length of the incident wave.
One of the objects of this invention is to provide an absorptive coating for electromagnetic wave energy having improved performance characteristics.
Another object of this invention is to provide an absorptive coating for electromagnetic wave energy capable of use in either an active or a passive mode.
Another object of this invention is to provide an absorptive coating for electromagnetic wave energy suitable for use over a wide range of incident wave frequencies.
Another object of this invention is to provide a coating for a metallic surface having a PN junction therein.
Still another object of this invention is to provide a coating in which a control voltage may be injected across a PN junction of a coating for a metallic surface.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.